The invention relates to a filter realized in a conductive case by means of mechanical structural elements, which filter is suitable for signal processing especially at microwave frequencies.
In present and future mobile communication networks more is required of filters than that their frequency responses comply with specifications. Low losses is a characteristic which is at least desirable in most filters. Low losses mean low attenuation in the pass band and easier matching. Good breakdown characteristics and power handling capacity are often required as well. For example, in WCDMA (Wideband Code Division Multiple Access) devices the strength of the electric field of the transmitted signal has instantaneous peaks which may cause breakdowns in the insulator. Strict requirements on the power handling capacity of a filter may be imposed especially in cases where a plurality of transmitted signals are summed. In the filter should not occur intermodulation to a harmful extent when signals at different frequencies travel through it. Furthermore, in the case of series manufactured filters that meet the requirements it is essential to bring the production costs down as much as possible.
There is a multitude of different known filter structures. The structure discussed in this description resemble to an extent filters consisting of resonators formed in a metal casing by means of mechanical structural elements. The resonators are usually arranged in a row so that they constitute a single block when viewed from the exterior. The most common resonator type is the coaxial quarter-wave resonator. Inter-resonator coupling, which is accomplished by means of auxiliary parts, is either capacitive or inductive. Coupling mechanism details may vary to a great extent. FIG. 1 shows an example of such a prior-art filter partly opened and disassembled. It comprises resonators, such as 110, 120 and 130, in a row. Each resonator comprises an inner conductor, such as 131, galvanically coupled at its lower end to the bottom plate 101 of the structure. The inner conductors may have extensions at their upper ends in order to increase the capacitance at the open end of the structure, thereby causing the resonator can be made shorter in the vertical dimension. Each resonator further comprises an outer conductor consisting of resonator partition walls, such as 103, and parts 102 of the side walls and end walls of the whole filter case. The structure includes a conductive cover so that the filter casing is closed. By way of example, the cover is provided with a screw 138 at resonator 130 for tuning the resonance frequency of that resonator. FIG. 1 shows by way of example one capacitive and one inductive coupling between the resonators. The capacitive coupling is between resonators 110 and 120 at their open ends where the electric field is relatively strong. For the capacitive coupling there is an aperture 107 in the wall 103 between the resonators 110 and 120. Conductive wings 114, 124 attached to the inner conductors of the resonators and directed towards the aperture add to the inter-resonator coupling capacitance. Input to the filter via connector IN is also capacitive. The inductive coupling is between resonators 120 and 130, near their short-circuited ends where the magnetic field is relatively strong. For the inductive coupling there is an element 125 shaped of conductive plate, which extends close to the inner conductors of said resonators and is grounded at suitable points. The element 125 produces mutual inductance between the resonators. A disadvantage of the structure described and like structures is the difficulty of filter tuning and the costs that follow therefrom. Moreover, the manufacturing prior to the tuning involves relatively high costs as well. Furthermore, there is a danger of generating harmful intermodulation results, especially if the structures employ more than one metal for the purpose of temperature compensation.
An object of the invention is to alleviate said disadvantages associated with the prior art. The structure according to the invention is characterized by that which is specified in the independent claim 1. Some preferred embodiments of the invention are specified in the other claims.
The basic idea of the invention is as follows: A metal casing houses a series of separated rigid conductive elements. Between the consecutive elements there is arranged a suitable capacitance which is adjustable within certain limits, if necessary. At least some of the conductive elements are connected with, or they include, a conductor less than half a wavelength long, short-circuited at the opposite end. Together with the casing connected to the signal ground the conductor constitutes a transmission line which, looking from the conductive element, is inductive at the operating frequencies. This way a signal path is provided in the filter, having capacitance in the longitudinal direction and inductance in the transversal direction, always between two capacitive parts. The structure is a high-pass type structure.
An advantage of the invention is that the filter structure according to it is simple in comparison with structures according to the prior art. This means savings in manufacturing costs. Another advantage of the invention is that the structure according to it is sturdy in comparison with the prior art. This means relatively good power handling capacity and reliability. A further advantage of the invention is that the structure according to it, in comparison with the prior art, has less boundaries that may cause harmful intermodulation.